1. Field of the Invention
The present invention relates to a control computer used in a hard disk drive, for example.
2. Description of the Related Art
Conventionally, a hard disk drive (HDD) includes a microcomputer serving as a control computer for controlling various circuits constituting the HDD such as a servo circuit, a read/write circuit of a head and a motor driver/motor controller. The control computer is a dedicated computer for executing predetermined control operations under the control of a control program.
Normally, the control program is fixedly stored in a read only memory (ROM) comprised of an IC memory. However, the control program must be changed depending on the product specification. For example, the hard disk drive must be customized depending on a customer's needs. For this reason, when the control computer is built in the HDD, the ROM is mounted on an IC socket. The use of IC socket permits the replacement of the ROM, to thereby change the control program.
However, the contact failure is apt to be caused when the ROM is inserted into the IC socket. Further, the cost is increased by the addition of the IC socket. Furthermore, a space for mounting the IC socket must be secured on the print circuit board. On the other hand, if the ROM is welded to the print circuit board without the IC socket, the ROM is not replaceable, which is impractical.
In order to solve the above problem, a method has been developed in which an electrically erasable programmable read only memory (EEPROM) is used in place of the ROM for storing the control program. The EEPROM is a nonvolatile and rewritable ROM which permits erase and rewrite operations. The EEPROMs are classified into a type in which the erase operation is performed in units of words, a type in which the erase operation is performed in units of blocks or rows, and a type in which erase operation is performed over the entire memory array, which is referred to as bulk erase. The EEPROM of bulk erase type is referred to as a flash EEPROM. Because of low cost, the flash EEPROM is used in place of the above described ROM. The use of the flash EEPROM permits the change of the control program without changing the memory itself. More specifically, the control program stored in the flash EEPROM may be replaced with a new control program supplied from a host computer, for example.
However, in order to rewrite the control program stored in the flash EEPROM, a dedicated program for rewriting the control program stored in the EEPROM to the new control program must be prepared. However, the rewrite program cannot be stored together with the control program in the flash EEPROM or in the EEPROMs other than the flash EEPROM. Because if the rewrite program is stored in the flash EEPROM together with the control program, the rewrite program will be erased when the erase operation is performed for replacing the control program with a new control program. Even if the EEPROMs other than the flash EEPROM are used, the rewrite program cannot be stored together with the control program because the rewrite program cannot be read out while writing the new control program in the same memory. For this reason, an additional ROM for storing the rewrite program is built in the HDD. However, such additional ROM for storing the rewrite program increases the cost and complicates the circuit arrangement.